The vision of a simultaneous efficient, durable, flexible, and low-cost light-emitting device is highly attractive from both an end-user and a producer perspective, but at the same time poses a significant scientific and technological challenge that remains effectively non-resolved as of now. Emerging fluorescent organic semiconductors can, in contrast to their more conventional inorganic counterparts, be processed by relatively simple methods at low temperatures, and are as such compatible with the employment of flexible substrates and low-cost roll-to-roll production. Accordingly, light-emitting devices based on organic semiconductors in the form of small molecules (SMs) or conjugated polymers (CPs) are attracting enormous scientific and commercial interest, with the current prime focus being aimed at the development of organic light-emitting diodes (OLEDs).
SM-based OLEDs exhibit an interesting attribute in that the properties of the active material can be tuned by controllable chemical doping of different layers within a multi-layer stack, and the performance of such appropriately designed devices have recently reached rather impressive levels. However, a notable drawback with SM-based OLEDs is that they are not typically amenable to solution processing and roll-to-roll fabrication, with a concomitant penalty in the simplicity and cost of fabrication. CP-based OLEDs, on the other hand, are compatible with a straightforward and low-cost solution processing of the polymeric active material, but suffer from the fact that doping is not realizable in practice. As a consequence, it is necessary to employ a low-work function and highly reactive cathode material in CP-OLEDs in order to attain good device performance, which has a negative impact on the device functionality from a fabrication and stability perspective.
An alternative, and frequently overlooked, organic light-emitting device is the light-emitting electrochemical cell (LEC). Its unique operation is based on that mobile ions are intimately intermixed with the organic semiconductor, and that these ions redistribute during device operation in order to allow for efficient electronic charge injection, transport and recombination. Moreover, CP-based LECs can be processed directly from solution using potentially cheap materials (based on common elements such as C, H, O, N, etc.), and accordingly offer most of the initially outlined requirements for the high-performance light-emitting device of the future. However, the significant drawback of the current generation of LECs, which rationalizes the as-of-yet limited interest from industry and academia, is related to a non-adequate operational lifetime.
There is prior art in the field of functional LECs with long lifetime, high power conversion efficiency, and/or flexible design.
US2008/0084158 and Shao, Y., G. C. Bazan, and A. J. Heeger, Long-lifetime polymer light-emitting electrochemical cells. Advanced Materials, 2007. 19(3): p. 365-+, discloses a significant operational lifetime for LEC devices of 100-1000 h. They disclose a dilute concentration of the electrolyte constituent (an ionic liquid) in the active material. These disclosures, however, are based on the theory that phase separation has a greatly limiting effect on the lifetime of the LEC. According to these disclosures, the improved lifetime is due to the fact that the two constituent materials in the active material (an ionic liquid and a conjugated polymer) form a single phase, since they are both hydrophobic.
Cao, Y., et al., Efficient, fast response light-emitting electrochemical cells: Electroluminescent and solid electrolyte polymers with interpenetrating network morphology. Applied Physics Letters, 1996. 68(23): p. 3218-3220, discloses a similar “single phase” approach when a surfactant is added to an active material mixture based on {MEH-PPV+PEO+LiCF3SO3}. They attained LEC devices with operational lifetimes of approximately 100 h at significant brightness. Importantly, the authors employ conventional high concentrations of the LiCF3SO3 salt and the ion-dissolving and ion-transporting PEO polymer.
In the herein exploited field of LEC devices with an active material mixture comprising a hydrophobic conjugated polymer blended with a dilute concentration of a hydrophilic electrolyte (here the salt KCF3SO3 blended with the ion-dissolving and ion-transporting solid-state solvent PEO), which form a phase-separated active material, there appears to be very little prior art.
deMello, J. C., et al., Ionic space-charge effects in polymer light-emitting diodes. Physical Review B, 1998. 57(20): p. 12951-12963. discloses a low concentration of salt in some of their LEC devices, but the concentration of the ion-dissolving and ion-transporting solid-state solvent was kept high, and the total electrolyte content was therefore high. This disclosure further focuses on the operational mechanism of the devices and did, for instance, not report any data on the operational lifetime.
State-of-the-art OLEDs with solely MEH-PPV as the active material and with a power conversion efficiency of less than or approximately equal to 2 lm/W was demonstrated in Spreitzer, H., et al., Soluble phenyl-substituted PPVs—New materials for highly efficient polymer LEDs. Advanced Materials, 1998. 10(16): p. 1340-+, Hsiao, C. C., et al., High-efficiency polymer light-emitting diodes based on poly 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene with plasma-polymerized CHF3-modified indium tin oxide as an anode. Applied Physics Letters, 2006. 88(3), Wu, X. F., et al., High-quality poly 2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene synthesized by a solid-liquid two-phase reaction: Characterizations and electroluminescence properties. Journal of Polymer Science Part a-Polymer Chemistry, 2004. 42(12): p. 3049-3054, and Malliaras, G. G., et al., Electrical characteristics and efficiency of single-layer organic light-emitting diodes. Physical Review B, 1998. 58(20): p. 13411-13414, It is noteworthy that this high-performance OLEDs employ a low-work function and thus highly reactive metal for the cathode (typically Ca), while the herein disclosed LEC devices with MEH-PPV in the active material exhibit a similar or better power conversion efficiency while employing a stable Al cathode.
In Santos, G., et al., Opto-electrical properties of single layer flexible electroluminescence device with ruthenium complex. Journal of Non-Crystalline Solids, 2008. 354(19-25): p. 2571-2574, there is disclosed the first flexible SM-based LEC, but with a very modest brightness level of 1 cd/m2 and a very low power efficiency of 0.003 lm/W.
Hence, there is a need for improved or alternative light-emitting devices, and in particular of such devices which have a longer operational life and/or which presents an increased versatility in terms of applications for use.